The black mamba (Dendroaspis polylepis) is a species of highly venomous snake belonging to the family Elapidae. It is native to parts of sub-Saharan Africa. First formally described by Albert Günther in 1864, it is the second-longest venomous snake after the king cobra; mature specimens generally exceed 2 m (6 ft 7 in) and commonly grow to 3 m (9.8 ft). Specimens of 4.3 to 4.5 m (14 to 15 ft) have been reported. It varies in colour from grey to dark brown. Juvenile black mambas tend to be more pale in colour than adults, and darken with age. Despite the common name, the black mamba is not black; the colour name describes rather the inside of its mouth, which it displays when feeling threatened.

The species is both terrestrial (ground-living) and arboreal (tree-living); it inhabits savannah, woodland, rocky slopes and in some regions, dense forest. It is diurnal and is known to prey on birds and small mammals. Over suitable surfaces, it can move at speeds up to 16 km/h (10 mph) for short distances. Adult black mambas have few natural predators.

In a threat display, the black mamba usually opens its inky-black mouth, spreads its narrow neck-flap and sometimes hisses. It is capable of striking at considerable range and may deliver a series of bites in rapid succession. Its venom is primarily composed of neurotoxins that often induce symptoms within ten minutes, and is frequently fatal unless antivenom is administered. Despite its reputation as a formidable and highly aggressive species, the black mamba attacks humans only if it is threatened or cornered. It is rated as least concern on the International Union for Conservation of Nature (IUCN)'s Red List of Threatened Species.

The first formal description of the black mamba was made in 1864 by German-born British zoologist Albert Günther. A single specimen was one of many species of snake collected by John Kirk, a naturalist who accompanied David Livingstone on the 1858–1864 Second Zambesi expedition. This specimen is the holotype and is housed in the Natural History Museum, London. The generic name of the species is derived from the Ancient Greek words dendron (δένδρον), "tree", and aspis (ἀσπίς) "asp", and the specific epithet polylepis is derived from the Ancient Greek poly (πολύ) meaning "many" and lepis (λεπίς) meaning "scale". The term "mamba" is derived from the Zulu word "imamba". In Tanzania, a local Ngindo name is ndemalunyayo ("grass-cutter") because it supposedly clips grass.

In 1873, German naturalist Wilhelm Peters described Dendraspis antinorii from a specimen in the museum of Genoa that had been collected by Italian explorer Orazio Antinori in what is now northern Eritrea. This was subsequently regarded as a subspecies and is no longer held to be distinct. In 1896, Belgian-British zoologist George Albert Boulenger combined the species Dendroaspis polylepis as a whole with the eastern green mamba (Dendroaspis angusticeps), a lumping diagnosis that remained in force until 1946 when South African herpetologist Vivian FitzSimons again split them into separate species. A 2016 genetic analysis showed the black and eastern green mambas are each other's closest relatives, and are more distantly related to Jameson's mamba (Dendroaspis jamesoni), as shown in the cladogram below.

The black mamba is a long, slender, cylindrical snake. It has a coffin-shaped head with a somewhat pronounced brow ridge and a medium-sized eyes. The adult snake's length typically ranges from 2 to 3 m (6 ft 7 in to 9 ft 10 in) but specimens have grown to lengths of 4.3 to 4.5 m (14 ft 1 in to 14 ft 9 in). It is the longest species of venomous snake in Africa and the second-longest venomous snake species overall, exceeded in length only by the king cobra. The black mamba is a proteroglyphous (front-fanged) snake, with fangs up to 6.5 mm (0.26 in) in length, located at the front of the maxilla. The tail of the species is long and thin, the caudal vertebrae making up 17–25% of its body length. The body mass of black mambas has been reported to be about 1.6 kg (3.5 lb), although a study of seven black mambas found an average weight of 1.03 kg (2.3 lb), ranging from 520 g (18 oz) for a specimen of 1.01 m (3 ft 4 in) total length to 2.4 kg (5.3 lb) for a specimen of 2.57 m (8 ft 5 in) total length.

Specimens vary considerably in colour, including olive, yellowish-brown, khaki and gunmetal but are rarely black. The scales of some individuals may have a purplish sheen. Individuals occasionally display dark mottling towards the posterior, which may appear in the form of diagonal crossbands. Black mambas have greyish-white underbellies. The common name is derived from the appearance of the inside of the mouth, dark bluish-grey to nearly black. Mamba eyes range between greyish-brown and shades of black; the pupil is surrounded by a silvery-white or yellow colour. Juvenile snakes are lighter in colour than adults; these are typically grey or olive green and darken as they age.

The number and pattern of scales on a snake's body are a key element of identification to species level. The black mamba has between 23 and 25 rows of dorsal scales at midbody, 248 to 281 ventral scales, 109 to 132 divided subcaudal scales, and a divided anal scale. Its mouth is lined with 7–8 supralabial scales above, with the fourth and sometimes also the third one located under the eye, and 10-14 sublabial scales below. Its eyes have 3 or occasionally 4 preocular and 2–5 postocular scales.